It is known that lead and many of its compounds are toxic. Prior to the 1970's, lead and other heavy metal-based pigments, such as cadmium-, chromium-, and arsenic-based pigments, were extensively used for the manufacture of paints. These paints constitute a major health hazard, and the removal of old paintwork and disposal of the paint residues is problematic and regulated. The removal of lead-based paint (LBP) by sanding or flame gun is prohibited is some jurisdictions. Sand blasting is permitted with appropriate health and safety measures and containment precautions in exterior environments.
Lead-based paints have been removed and treated utilizing calcium silicate-containing sand blasting grit presently sold under the trade name Blastox. Similar metal-sorbing abrasives used for sand blasting and containing phosphates are sold under the trademark Leadx. When subsequently wetted, the residues from LBPs stripped with the alkali calcium silicate-containing abrasives form a stabilized complex of lead carbonate and lead silicate in a calcium-rich cement-like matrix. Though no untreated control study was undertaken, calcium silicate LBP residues were shown to reduce leachability of lead by TCLP to less than 5 mg/litre, achieving “non-hazardous” classification for landfills, apparently through dilution with the abrasive, the action of its intrinsically high alkalinity (greater than pH 10), and it cementatious nature. However, the nature of the calcium silicate matrix is such that it is potentially susceptible to pH reduction and destabilization through the action of atmospheric carbon dioxide. Thus, the residue, whilst considered non-hazardous by virtue of its low leachable lead, is not necessarily entirely non-hazardous, by virtue of its high alkalinity. Furthermore, the residue is not non-toxic by ingestion or inhalation. Residues from calcium silicate sand blasting are fine. To avoid their becoming wind-blown, they must be contained using conventional blanketing techniques or by spraying as slurry. Also, operatives must be protected from breathing or ingesting fine dusts and aerosols.
Paint strippers are not designed “to fix” lead and other metal pigments found in paints, but simply facilitate removal of the paint from a surface. The resulting residue (paint-strippings) contains toxic metals in a potentially leachable form, and must be disposed of in accordance with applicable government regulations, often at substantial expense.
Modern paint strippers are typically blended formulations based on one or more stripping agents, such as an organic solvent. Methylene chloride-based strippers are particularly widely used. Other ingredients commonly found in paint strippers include cosolvents, activators and corrosion inhibitors, evaporation retarders, thickeners, emulsifiers, wetting agents, and detergents. Chemically, the ingredients found in paint strippers include chlorinated and unchlorinated hydrocarbons, other organic solvents, organic oils, water, alcohols, amines, esters, lactones, pyrrolidones, phenols, organic acids, sulphonic acids, peracids, and peroxides. The components are maintained in a homogeneous suspension with or without the addition of an emulsification agent. Other agents may be added, e.g. humectants, antifoams, hardness regulators and couplers to regulate the polarity of acidic components.
When applied to a dried, painted surface, the paint stripper causes swelling of the surface coating (paint) polymer, and/or breaks the chemical bonds of the coating polymer so that it can be easily mechanically removed by a scraper, brush, or similar device. Some paint strippers contain viscosity-modifying agents such as viscous oils, colloidal silica, hydroxyethyl, hydroxymethyl, or hydroxypropyl cellulose ethers, polyethylene waxes, and polyvinyl alcohol, so that when the stripper is applied to a vertical or other surface it remains in contact with the paint for sufficient time to effect swelling and facilitate paint removal.
Numerous patents have issued for paint strippers. Although some patents address the need for more “environmentally friendly” strippers, by using non-chlorinated solvents or by lowering the number of volatile organic compounds (VOCs), they typically ignore the greater environmental threat posed by lead and other heavy metal pigments contained in the paint itself. U.S. Pat. No. 6,465,405 (Vitomir) discloses the use of peroxide-containing paint stripper compositions having an ambient pH of 2 to 3. These strippers may contain a chelating agent such as phosphonic acids, citric acid, EDTA, etc., at about 0.5 to 4% by weight of the formulation. However, such chelating agents are not heavy metal remediation agents capable of reducing the water solubility of heavy metals at the pH's prevailing in these formulations. Rather, the chelating agents function as hardness regulators, i.e., stabilizers against the use of hard water. Calcium in hard water can interact with the organic acids in the composition, leading to de-stabilization of the emulsified stripper.
Although EDTA has been used for some soil remediation processes and for treatment of lead poisoning, it is not used to reduce the solubility of soluble lead, nor is it used to reduce the toxicity of lead compounds. EDTA is applied to lead systems to form a soluble lead chelate complex. In the case of lead poisoning, the administration of EDTA solubilize the lead in the body to allow it to be excreted from the body in urine and bile. In the case of soil remediation , EDTA is used to solubilize lead such that it may be washed from contaminated soil.
Given the amount of lead-based and similar metal-pigmented paint in place in buildings and other surfaces around the world, and in view of the deficiencies of existing paint strippers, there is a substantial need for an improved paint stripper capable of remediating lead and other heavy metals found in paints.